The Epworth Sleepiness Scale: Validation of One-Dimensional Factor Structure in a Large Clinical Sample

The Epworth Sleepiness Scale (ESS) is used by clinicians and researchers to determine level of daytime sleepiness. The number of factors included in the scale has been debated. Our study objective was to clarify the dimensionality of the ESS using a large clinical sample.

A retrospective cohort study included all patients presenting for care in a tertiary care sleep disorders center who answered all items on the ESS from January 8, 2008 to September 28, 2012. Dimensionality was assessed using scree plot, eigenvalues, factor loadings, principal factor analysis, and confirmatory factor analysis. Multigroup confirmatory factor analysis (MGCFA) evaluated dimensionality within 10 subgroups of clinical interest.

The mean age of the 10,785 study participants was 50 (± 15) years with 49% female, and 81% white. The one-factor solution explained 63% of the variability in responses with high factor loadings (> .67 for all 8 items). The scree plot identified one factor with eigenvalue > 1. Results of confirmatory factor analysis demonstrated a one-factor solution had acceptable goodness of fit as assessed by root mean square error of approximation of .094 (90% confidence interval: .089-.099). MGCFA confirmed measurement invariance within all 10 demographic and clinical subgroups.

Our study confirmed the unidimensionality of the ESS in a large diverse clinical population. Results from this study can be used to justify the interpretation of the ESS within clinical populations, and supports valid comparisons between groups based on the ESS. Future studies are warranted to further understand the items comprising the ESS and potentially eliminate redundant items for increased efficiency in clinical settings.